1. Technical Field
This invention relates generally to a system for trapping sulfur carried in exhaust gases from an internal combustion engine, and more particularly to such a system that prevents sulfur carried in the exhaust gases from passing through a sulfur-sensitive component of an exhaust gas aftertreatment system.
2. History of Related Art
Internal combustion engines generate oxides of nitrogen (NOx) emissions as the result of high temperature combustion. NOx emissions are known to be responsible for ground level ozone production through photocatalytic processes in the atmosphere. Regulations have been in place for over two decades to reduce NOx emissions from internal combustion engines. Hydrocarbons (HC) and carbon monoxide (CO) are also regulated as harmful emissions. Despite a large increase in the number of internal combustion engines in use, the atmospheric levels of HC and CO have been successfully reduced. However, NOx emissions have remained at approximately the same levels and have even increased in areas of high automobile usage, such as large metropolitan areas. As a result, proposed new regulations call for dramatic reductions in NOx emissions. A significant amount of research is currently being conducted into NOx catalysis and NOx traps in an attempt to find solutions that further reduce tailpipe NOx emissions. However, sulfur compounds (primarily in the form of SO2 derived from naturally existing sulfur compounds in fuels which are oxidized to SO2 through the combustion process) are recognized as primary inhibitors of NOx system efficiencies. Currently, some automotive catalytic systems require the use of low-sulfur gasoline fuel to prevent poisoning of the catalyst material used to reduce the NOx emissions. Oil companies are continually pressured to find sources of low-sulfur crude oil and significantly reduce the sulfur content of refined fuels, at increasing expense to the oil companies, costs that may ultimately be passed on to the end user.
Three-way conversion (TWC) catalysts are used in many vehicles to reduce emissions of NOx, CO, HC, and particulate matter. U.S. Pat. No. 5,057,483, issued Oct. 15, 1991 to Chung-Zong Wan for Catalyst Composition Containing Segregated Platinum and Rhodium Components, describes a catalyst having a bulk metal oxide, such as bulk nickel oxide, as one component of the catalytic system to suppress hydrogen sulfide (H2S) emissions. Likewise, U.S. Pat. No. 5,490,977 issued Feb. 13, 1996 to Chung-Zong Wan for Removal of CO Hydrocarbons, and NOx with catalyst containing Platinum Rhodium, describes a three-way conversion (TWC) catalyst containing a metal oxide which is effective for the suppression of hydrogen sulfide. The metal oxide is preferably nickel oxide. Sulfur compounds, which result from the combustion of sulfur in commonly used fuels, are converted to hydrogen sulfide during transient fuel-rich operating conditions such as idling and acceleration. Hydrogen sulfide (H2S) has a characteristic foul odor, often described as smelling like xe2x80x9crotten eggsxe2x80x9d. The metal oxide H2S suppressor described in the referenced patent temporarily traps any hydrogen sulfide, thereby delaying the discharge of hydrogen sulfide from the catalyst until fuel-lean combustion operation is established whereupon an oxygen-rich environment is established in the exhaust gases and the hydrogen sulfide is oxidized to various sulfur compounds.
In both of the above-described TWC catalysts, hydrogen sulfide compounds are temporarily stored, but subsequently passed, in one form or another, through the NOx catalytic system. Therefore, over time, the active surfaces of the catalyst, designated for NOx reduction, become less effective due to the presence of sulfur compounds trapped on their surfaces.
The present invention is directed to overcoming the problems set forth above. It is desirable to have an exhaust gas aftertreatment system in which sulfur compounds are deleted from the exhaust gases passing through a sulfur-sensitive emission reduction device (SSERD) of the system. It is also desirable to have such a system in which sulfur compounds carried in the exhaust gas stream from the engine are initially trapped, stored, and then discharged from the trap by diverting the exhaust gas stream around the sulfur-sensitive emission reduction device (SSERD) during purging of the trap. It is also desirable to have such a system in which a lean sulfur trap is adapted to store sulfur compounds during lean fuel-air mixture combustion, and then when saturated, discharge the stored sulfur compounds to a rich fuel-air mixture sulfur trap by the injection of a reducing agent, such as a hydrocarbon fuel, upstream of the lean sulfur trap. It is also desirable to have such a system in which a first sulfur trap effectively carries out a reaction with sulfur compounds in an oxidizing atmosphere, and a second sulfur trap which effectively carries out the reaction of sulfur compounds in a reducing environment. The second sulfur trap is in selective fluid communication with the first sulfur trap. The second sulfur trap is adapted to receive exhaust gases containing hydrogen sulfide from the first sulfur trap. The hydrogen sulfide is reacted with a metal oxide in the second sulfur trap to form a metal sulfide and water, temporarily store the metal sulfide, and then oxidize the metal sulfide in the presence of exhaust gases that are substantially free of sulfur and have an excess of oxygen. Exhaust gases discharged from the second sulfur trap contain sulfur dioxide.
In accordance with one aspect of the present invention, a sulfur trap system for use in an exhaust gas aftertreatment system containing a sulfur-sensitive emission reduction device (SSERD) includes a first sulfur trap, a second sulfur trap, a means for selectively injecting a reducing agent into the exhaust gases prior to the exhaust gas being received by the first sulfur trap, and a means for selectively directing the exhaust gases discharged from the first sulfur trap to either the SSERD or the second sulfur trap. Sulfur dioxide is oxidized in the first sulfur trap, wherein sulfur is temporarily stored as sulfite and sulfate species. During normal operation, the exhaust gas discharged from the first sulfur trap is substantially free of sulfur, and the sulfur-free exhaust gases are accordingly directed through the SSERD of the aftertreatment system.
Other features of the sulfur trap system embodying the present invention include the first sulfur trap having a Group VIII metal catalyst such as platinum, palladium or rhodium. Other features include the second sulfur trap having a metal oxide, such as nickel oxide, germanium oxide, copper oxide, or manganese oxide. Still another feature includes the means for injecting a reducing agent into the exhaust gases being a fuel injector in fluid communication with a source of hydrocarbon fuel. Yet another feature includes the means for selectively directing exhaust gases discharged from the first sulfur trap to either the SSERD or the second sulfur trap being an exhaust gas flow diverter valve that is selectively moveable between a first position at which the exhaust gases are directed to the SSERD, and a second position at which the exhaust gases discharged from the first sulfur trap are directed to the second sulfur trap.
In another aspect of the present invention, a method for preventing sulfur dioxide from passing through a sulfur-sensitive emission reduction device (SSERD) in an exhaust gas aftertreatment system of an internal combustion engine that is adapted to generally operate in a lean fuel-air combustion mode (e.g. diesel or lean-burn gasoline), and discharge exhaust gases from the engine that contain excess oxygen remaining after combustion of the lean fuel-air mixture, include conducting the exhaust gases discharged from the internal combustion engine to a first sulfur trap disposed in direct fluid communication with an exhaust manifold of the engine. Sulfur dioxide carried in the exhaust gases discharged from the engine are oxidized in the first sulfur trap to form sulfur trioxide (SO3), which is then converted to sulfite and sulfate species and temporarily stored in the first sulfur trap. The exhaust gases discharged from the first sulfur trap are thus substantially free of sulfur and are conducted to the SSERD. The point at which the first sulfur trap is substantially saturated with sulfite and sulfate species is determined, and the flow of exhaust gas to the sulfur-sensitive emission reduction device (SSERD) is interrupted and directed to the second sulfur trap. A reducing agent is then injected into the exhaust gases prior to the exhaust gases being received by the first sulfur trap whereby the sulfite and sulfate species stored in the first sulfur trap are reduced to hydrogen sulfide and conducted to the second sulfur trap by the exhaust gases directed to the second sulfur trap. In the second sulfur trap, the hydrogen sulfide contained in the exhaust gases is reacted with a metal oxide to form a metal sulfide and water, the metal sulfide being temporarily stored in the second sulfur trap. The point at which the sulfite and sulfate species stored in the first sulfur trap are substantially completely reduced to hydrogen sulfide, is determined and the injection of the reducing agent into the exhaust gases is interrupted. The metal sulfide temporarily stored in the second sulfur trap is then oxidized, in the presence of the exhaust gases discharged from the first sulfur trap that are substantially free of sulfur and have an excess of oxygen carried therein, to form sulfur dioxide. Exhaust gases thus containing sulfur dioxide are discharged from the second sulfur trap. When the oxidation of metal sulfide in the second sulfur trap is substantially complete, the flow of exhaust gas to the second sulfur trap is interrupted, and redirected to the SSERD.